WEEK 2: Ametropia and Objective Refraction Flashcards
What is The optical differences amongst emmetropia and ametropia?
Emmetropia is the refractive status whereby when an eye with accommodation fully relaxed focusses parallel light rays (e.g. from a distant object) onto the retina
Ametropia refers to any status whereby an eye, with accommodation fully-relaxed, does not focus collimated light rays onto the retina
The relationship between the magnitude of an eye’s ametropia, the location of its focal point/s and relative size of retinal blur image
Myopia - focal point is in front of the retina, the further the focal point is from the retina, the bigger the blur cirlce.
Hyperopia - focal point is in behind of the retina, the further the focal point is from the retina, the bigger the blur cirlce.
Astigmatism -
The formation and orientation of the focal lines and Circle of least Confusion with regular astigmatism
Light entering the eye/lens results in the formation of two non-coincident focal points. In the vertical meridian, light comes to a focus and forms a FOCAL LINE with orientation perpendicular to the in the plane in focus i.e. horizontal focal line. In the horizontal meridian, light comes to a focus to form a FOCAL LINE here with orientation perpendicular to the plane in focus i.e. vertical focal line.
The classification of different categories of Astigmatism
▪ With-the-rule: max refractive power along 90ᵒ ± 30ᵒ (negative cyl axis = 180ᵒ ± 30ᵒ)
▪ Against-the-rule: max refractive power along 180ᵒ ± 30ᵒ (negative cyl axis = 90ᵒ ± 30ᵒ)
▪ Oblique: max refractive power in the remaining meridians (not with or against)
• Irregular astigmatism: Principal meridians are not perpendicular
• Simple myopic/hyperopic astigmatism: One focal line falls on the retina, with the other in front/behind the retina
• Compound myopic/hyperopic astigmatism: Both focal lines fall in front/behind the retina
• Mixed astigmatism: One focal line falls in front of the retina, the other behind the retina
How spectacle/contact lenses enable clear vision of an object not positioned at the ametropic eye’s far-point
Spectacle and contact lenses ‘correct’ ametropia by placing the image at an eye’s FP
i.e. the spectacle/contact lens’ secondary focal point should coincide with an eye’s FP
With the image (of the object) at the eye’s FP, it is conjugate to the fovea. Hence, the eye focussed and clear
Where is the Circle of least Confusion with regular astigmatism
Image formation of a point source is least distorted at a position dioptrically mid-way between the two focal lines. The Circle of Least Confusion is formed at this point
What is the interval of sturm?
The dioptric distance between the two focal lines (which correspond to the extremes of the refractive component of the eye/lens) is called the Interval of Sturm.
What are the signs and symptoms of Myopia
Signs:
Reduced distance visual acuity
Normal near visual acuity
Symptoms:
Constant distance blur
Squinting
Short working distance preferred
What are the signs and symptoms of Hyperopia
Signs:
Reduced amplitude of accommodation
Reduced near visual acuity
Usually normal distance visual acuity
Symptoms:
Asthenopia and headaches
Blur at near
Often long working distance
What are the signs and symptoms of Astigmatism
Signs:
Reduced distance and near visual acuity
Symptoms:
Constant blur at distance and near
Squinting
Asthenopia and headaches
Difficulty adapting to spectacle Rx changes
What are the signs and symptoms of Presbyopia
Signs: Decreasing near VA
Decreasing amp Acc
Increased lag of accommodation
Symptoms:
Blur at near
Asthenopia with near work
Longer than usual working distance
What is objective refraction?
Determine an eye’s refraction based solely on the optics of the eye
What is practitioner dependent objective refraction?
Practitioner dependent: Determination made without patient input, however practitioner interpretation required e.g. retinoscopy, keratometry
What is practitioner independent objective refraction?
Practitioner-independent: Determination made without patient or practitioner input/interpretation e.g. auto-refraction, photo-refraction
List 3 reasons why we do Objective refraction
• Useful starting point for subjective refraction
• Confirmation of endpoint of subjective refraction
• Malingerers
• If subjective responses are limited
• Children
• Language barriers
Cognitive disability
List the 4 reason to do retinoscopy
- More reliable than autorefraction in children (unless cycloplegia)
- Allows rudimentary assessment of
a. Ocular media transparency
b. Anomalies in refractive components of the eye (eg corneal ectasias) - Dynamic, can be used in accommodation assessment
Inexpensive, portable
- Allows rudimentary assessment of
What is a KEY CONCEPT when thinking about retinoscopy?
The FAR POINT of the eye is the location from which emerging light will focus on the retinal plane.
Are you myopic? Take a moment to find your far point….
Describe the principle of Neutralisation with retinoscopy
When the far point of the system (natural far point of the eye plus correcting lenses) aligns with the sight
hole of the retinoscope, the patients pupil will appear filled with ret-reflex. This is termed “neutralisation”.
No movement of the retinoscope beam reflex is what we are aiming for – this means we have neutralised the refractive error
What is the aim of Static Retinoscopy and how is it achieved?
Aim: Neutralisation of ret-reflex to determine the eye’s optical refraction
- Achieved when the rays from the retina (object) come to a focus where the retinoscope sight-hole. When neutralised, the ret-reflex appears to fill the pupil, as all the rays exiting the pupil are focussed at the retinoscope sight-hole
When does “Neutral” retinoscopy reflex occur?
“Neutral” retinoscopy-reflex occurs when the focal point of the retinal image^ is at the
retinoscope sight hole AKA the refractive error has been neutralised IN THAT MERIDIAN
Reminder
- With a streak retinoscope, you are scoping (attempting to neutralise) the meridian perpendicular to the orientation of the retinoscope slit(ie the direction you are sweeping the beam)
Sweep your retinoscope perpendicular to the retinoscope slit orientation
Explain what is a split reflex
Due to optical aberrations. Away from the visual axis, the refractive error will differ from centrally. Concentrate on neutralising the central retinoscopy reflex.
You are performing static retinoscopy, with the retinoscope’s collar down. You use your ret lens to compensate for your working distance. The following reported lenses do not require WD compensation.
With the retinoscope beam positioned along 120 degrees the ret-reflex was neutralised with +1.00DS. Along the other principal meridian, the ret reflex was neutralised with -0.50DS
a. Assuming the patient has regular astigmatism, which meridian has been neutralised with +1.00DS?
b. Assuming the patient has regular astigmatism, what is the patient’s net retinoscopy prescription, in sphero negative cylinder form?
Assuming the patient has regular astigmatism, which meridian has been neutralised with +1.00DS?
Beam oriented at 120 degrees will move along 30 degrees
Answer: The 30 degree meridian has been neutralised.
Assuming the patient has regular astigmatism, what is the patient’s net retinoscopy prescription, in sphero negative cylinder form?
+1.00 @30 and -0.50 @ 120
Most positive power: +1.00DS
Difference between the two powers: 1.50DS
Final Rx is: +1.00/-1.50 x 30
